1. Field of the Invention
This invention relates to field effect transistors (FETS) and in particular to power FETs, and especially to those made of silicon carbide, such as silicon carbide MESFET, having an improved structure which permits them to operate more efficiently at high power at radio frequency (RF) frequencies.
2. Description of the Prior Art
A FET device that can deliver RF power with high gain and satisfactory efficiency is a requirement for many system applications. A device commonly used for these applications is the metal-semiconductor field effect transistor (MESFET). The MESFET is an attractive microwave device for implementation in wide bandgap semiconductor due to its simple structure, ease of fabrication and excellent RPF performance.
Prior to the availability of large diameter, high-quality, monocrystalline silicon carbide (SiC) substrates, gallium arsenide (GaAs) had been preferred over silicon (Si) materials for high frequency applications under extreme conditions. This is, in part, because GaAs possesses a wider energy bandgap, higher peak electron velocity, greater radiation tolerance and a wider operating temperature range than Si. However, the thermal conductivity of GaAs is low (0.46 W/cm-.degree.C.) as compared to Si (1.5 W/cm-.degree.C.). This characteristic limits the usefulness of GaAs in high-power, high-density circuits operating under extreme temperatures. While GaAs provides a semi-insulating substrate which reduces device and interconnection capacitance, and makes the material ideal for an all-ion-implanted planar device technology, it lacks a high-quality native oxide. Without this oxide, reliable surface passivation is usually unavailable, causing GaAs MESFET devices to exhibit less-than-desirable performance in some high-frequency, high-power applications such as RF systems.
In U.S. Pat. No. 5,043,777 ('777 patent), Sriram sought to reduce the surface layer current flow in GaAs MESFETs, which he believed to be associated with free arsenic at the device surface, by growing an undoped GaAs surface layer upon, and lattice-matched to, the free surface of the n channel layer. This undoped lattice-matched surface layer extended at least between the source and drain n+ regions, and was intended to separate the surface charges from the MESFET active layers, thus minimizing their influence on device characteristics. one mechanism by which the undoped layer is thought to work is to increase the pinch-off voltage in the gate-drain region without producing any additional amount of undepleted charge as compared to prior GaAs device structures. In the '777 patent, Sriram suggested that these surface effects may be common to those MESFETs and high electron mobility transistors made of elements from groups III-V of the periodic table.
Silicon carbide (SiC) is a wide energy bandgap (3 eV) semiconductor which is an attractive material for fabrication of RF power MESFETs due to its unique combination of high saturated electron velocity (2.0.times.10.sup.7 cm/s), high junction breakdown voltage (5.times.10.sup.6 V/cm), high thermal conductivity (5 W/cm-.degree.C.) and broad operating temperature range (1100.degree. C.). Indeed, the thermal conductivity and breakdown voltage values for SiC are an order of magnitude greater than conventional semiconductor materials, such as Si, GaAs and Indium phosphide. In addition, the energy band gap and, therefore, the maximum operating temperature range of SiC, is at least twice that of conventional semiconductors.
These features are important to systems such as radar, which demand very high RF power requirements of system components, and avionics, which require stable device behavior under extreme operating temperatures. Also, because the sic crystal lattice is inherently tolerant to radiation, the operations of devices fabricated from SiC are less susceptible to the effects of radiation than conventional semiconductor materials. As a result, SiC devices are useful in high radiation environments, including nuclear system and space applications.
Silicon carbide MESFETs with excellent DC and small-signal characteristics have been fabricated which showed drain currents greater than 200 mA/mm and a breakdown voltage of greater than 100 volts. Such devices have also developed a small-signal gain of around 12 dB at 2 GHz. However, when operated as a power device, the power output was significantly lower than that predicted from the DC current-voltage (I-V) characteristics and, under pulsed current conditions, was actually less than DC current values. The origin of this untoward effect is unknown but is believed to be due to certain phenomena, collectively called "surface effects", which may include, for example, current flow in the surface of the device between the drain and the gate of the MESFET. This current flow can cause transconductance dispersion and other parasitic surface effects, as well as other unknown effects. Despite the clearly advantageous properties of SiC MESFETs, these devices could not successfully be fabricated heretofore for high RF power operations under extreme conditions.
There is a need, therefore, for a SiC MESFET which can operate at high power in the RF bands with sufficient efficiency.